Fiber bragg grating central wavelength demodulation method and system based on edge filtering

Abstract

The invention relates to the technical field of fiber bragg grating sensors, and discloses a fiber bragg grating central wavelength demodulation system based on edge filtering, which comprises an MG-Y tunable semiconductor laser, a first-stage optical splitter, a second-stage optical splitter and a fiber bragg grating sensor. The MG-Y tunable semiconductor laser is connected with a first-stage optical splitter, the first-stage optical splitter is connected with a plurality of second-stage optical splitters, each second-stage optical splitter is sequentially connected with a plurality of fiber grating sensors, each second-stage optical splitter is connected with a photodiode, each photodiode is connected with an amplification circuit, and the amplification circuit is connected with a plurality of optical fiber grating sensors. Each amplification circuit is connected with one ADC chip, and the ADC chips are connected with the master control FPGA / single-chip microcomputer. In a system for grating demodulation based on the MG-Y scanning laser, full-wave band scanning and local scanning are combined, large-step coarse scanning and small-step accurate scanning are combined, the demodulation speed and the demodulation precision are improved, the speed requirement of an analog chip is reduced, and therefore the cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber grating sensors, and particularly relates to a method and system for demodulating the central wavelength of a fiber grating based on edge filtering. Background Technique

[0002] Fiber grating sensors are based on the Bragg grating principle, and obtain sensing information by modulating the fiber Bragg wavelength with external physical parameters. They have the advantages of high sensitivity, anti-electromagnetic interference, corrosion resistance, and stable operation in extreme environments. They are widely used in the health monitoring of infrastructure such as bridges, dams, and tunnels, the monitoring of oil and gas pressure and temperature in long-distance pipelines, the icing and temperature monitoring of high-voltage power cabinets and transmission lines, and the temperature and strain monitoring of oil wells and mine shafts. The fiber grating demodulation technology based on an MG-Y tunable semiconductor laser is the most common fiber grating demodulation technology in the industry at present. It has the advantages of a wide wavelength tuning range, fast tuning speed, high power stability, and high side mode suppression ratio.

[0003] During the wavelength demodulation process, the scanning step size is required to be as small as possible to ensure the accuracy of central wavelength demodulation. The tuning range of the current mainstream MG-Y tunable semiconductor laser generally exceeds 40 nm. Therefore, an overly small scanning step size will seriously increase the number of scanning points and scanning frequency of the MG-Y tunable semiconductor laser. Therefore, the traditional fiber grating demodulation system based on an MG-Y tunable semiconductor laser has the following deficiencies:

[0004] 1. Since the MG-Y tunable semiconductor laser requires 5-way current control at the same time, and the cost of the analog chip increases exponentially with the increase of the bandwidth, high-frequency current tuning greatly increases the system cost.

[0005] 2. As the system bandwidth of the hardware analog circuit increases, the noise increases and the signal-to-noise ratio decreases, which will reduce the accuracy of wavelength demodulation.

[0006] 3. The demodulation accuracy of the traditional reflection spectrum maximum value detection method depends heavily on the scanning step size, resulting in insufficient demodulation accuracy of the central wavelength. Summary of the Invention

[0007] The present invention provides a method and system for demodulating the central wavelength of a fiber grating based on edge filtering. In a system for grating demodulation based on an MG-Y scanning laser, a combination of full-band scanning and local scanning, and a combination of large-step coarse scanning and small-step precise scanning are adopted to improve the demodulation speed and accuracy, and reduce the speed requirement of the analog chip, thereby reducing the cost.

[0008] The present invention provides a fiber Bragg grating central wavelength demodulation system based on edge filtering, which includes an MG-Y tunable semiconductor laser, a first-stage optical splitter, a second-stage optical splitter, a fiber Bragg grating sensor, a photodiode, an amplification circuit, an ADC chip, and a main control FPGA / microcontroller;

[0009] The MG-Y tunable semiconductor laser is connected to the first-stage optical splitter, the first-stage optical splitter is connected to a plurality of second-stage optical splitters, each second-stage optical splitter is sequentially connected to a plurality of fiber Bragg grating sensors, each second-stage optical splitter is connected to a photodiode, each photodiode is connected to an amplification circuit, each amplification circuit is connected to an ADC chip, and a plurality of ADC chips are all connected to the main control FPGA / microcontroller;

[0010] The right current, left current, phase current, SOA current, and Gain current are used to control the MG-Y tunable semiconductor laser for wavelength output, so that the output wavelength steps from the starting wavelength to the ending wavelength stably at a fixed step size and fixed power; the output wavelength passes through the first-stage optical splitter and the second-stage optical splitter and enters the external fiber Bragg grating sensor. When the output wavelength matches the fiber Bragg grating central wavelength of the fiber Bragg grating sensor, the output wavelength light is reflected back to the second-stage optical splitter by the Bragg grating of the fiber Bragg grating sensor, and is output to the photodiode through another port of the second-stage optical splitter. The photodiode converts the reflected optical signal into a current signal, which enters the ADC chip through the amplification circuit, and then the main control FPGA / microcontroller performs data acquisition and demodulates the fiber Bragg grating central wavelength. During the wavelength demodulation process, optical wavelength scanning is performed, including the initial scan, the second and subsequent scans.

[0011] Further, during the initial scan, the main control FPGA / microcontroller controls the right current, left current, phase current, SOA current, and Gain current, so that the right current, left current, phase current, SOA current, and Gain current control the MG-Y tunable semiconductor laser to perform optical wavelength scanning, and the wavelength is scanned from the starting wavelength at a fixed step until the ending wavelength is scanned.

[0012] Further, when the difference between the wavelength output by the MG-Y tunable semiconductor laser and the fiber Bragg grating central wavelength is less than the threshold value, the reflected light of the fiber Bragg grating sensor begins to increase. When the output wavelength of the MG-Y tunable semiconductor laser is exactly equal to the fiber Bragg grating central wavelength, the reflected light of the fiber Bragg grating sensor reaches the strongest. When the output wavelength of the MG-Y tunable semiconductor laser continues to increase, the reflected wavelength of the fiber Bragg grating sensor begins to decrease. At this time, the corresponding photodiode current and ADC chip voltage have achieved a process of first increasing and then decreasing, so that the fiber Bragg grating reflection spectrum is collected and the initial scan is completed.

[0013] Further, after obtaining the fiber Bragg grating reflection spectrum, the central wavelength of the fiber Bragg grating is calculated according to the centroid method, and the formula is as follows:

[0014]

[0015] Wherein, WAVE 中心波长 represents the central wavelength of the fiber Bragg grating to be calculated; a to b represent the sequence numbers from the start of scanning corresponding to the wavelength points recorded when the spectral amplitude exceeds the demodulation threshold; RANGE n represents the spectral peak value of the reflected wavelength corresponding to each point when scanning from point a to point b; WAVE step represents the scanning step of the MG-Y tunable semiconductor laser; WAVE start represents the actual wavelength value when the MG-Y tunable semiconductor laser starts scanning.

[0016] Further, after the initial scan is completed, the second and subsequent scans are started. The range of the calculated central wavelength of the fiber Bragg grating ± the set wavelength is used, and the scan is performed at the set step size. According to the wavelength sequence number and the reflected spectrum value during the scan, the central wavelength of the fiber Bragg grating is calculated according to the centroid method of the initial scan.

[0017] Further, the range of the central wavelength of the fiber Bragg grating ± the set wavelength is determined according to the bandwidth of the fiber Bragg grating sensor.

[0018] The present invention also provides a method for demodulating the central wavelength of a fiber Bragg grating based on edge filtering. Based on the above-mentioned system for demodulating the central wavelength of a fiber Bragg grating based on edge filtering, the method includes:

[0019] S1. The right current, left current, phase current, SOA current, and Gain current control the MG-Y tunable semiconductor laser to output WAVE start wavelength;

[0020] S2. The right current, left current, phase current, SOA current, and Gain current control the output of the current wavelength + WAVE step wavelength;

[0021] S3. Determine whether the output wavelength is less than the termination wavelength;

[0022] S4. If the output wavelength is less than the termination wavelength, return to step S2;

[0023] S5. If the output wavelength is greater than or equal to the termination wavelength, calculate the central wavelength of the fiber Bragg grating using the centroid method;

[0024] S6. The right current, left current, phase current, SOA current, and Gain current control the output of the central wavelength of the nth grating - the set wavelength;

[0025] S7, Right Current, Left Current, Phase Current, SOA Current, Gain Current control the current wavelength + WAVE step Wavelength output;

[0026] S8, Determine whether the output wavelength is less than the central wavelength of the nth grating + the set wavelength;

[0027] S9, If the output wavelength is less than the central wavelength of the nth grating + the set wavelength, return to step S7;

[0028] S10, If the output wavelength is greater than or equal to the central wavelength of the nth grating + the set wavelength, calculate the central wavelength of the nth grating using the centroid method;

[0029] S11, Return to step S6 until all wavelengths are calculated.

[0030] The beneficial effects of the present invention are as follows:

[0031] The present invention abandons the full-band scanning for each time of the MG-Y laser. Only during the first scan after the demodulation system receives the instruction, a large step size and full-band scanning are performed to roughly determine the central wavelength of each fiber grating. During the subsequent scans, a small step size precise scan near the central wavelength is performed. By combining full-band scanning and local scanning, the scanning speed of the system is faster, and without upgrading the hardware, the demodulation speed is greatly improved; by combining large step size rough scanning and small step size precise scanning, the demodulation accuracy of the demodulation system can be greatly improved, reducing the speed requirement of the analog chip, thereby reducing the cost. Description of the Drawings

[0032] Figure 1 It is a schematic structural diagram of the fiber grating central wavelength demodulation system based on edge filtering of the present invention.

[0033] Figure 2 It is a schematic flow diagram of the fiber grating central wavelength demodulation method based on edge filtering of the present invention.

[0034] The realization, functional characteristics and advantages of the purpose of the present invention will be further described in conjunction with the embodiments and with reference to the drawings. Specific Embodiments

[0035] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0036] The present invention is applied to the fiber grating demodulation module and the host, including a low-speed fiber grating demodulation module / host, a medium-speed fiber grating demodulation module / host, and a high-speed fiber grating demodulation module / host.

[0037] Such as Figure 1As shown in the figure, the present invention provides a fiber Bragg grating central wavelength demodulation system based on edge filtering, which includes an MG-Y tunable semiconductor laser, a first-stage optical splitter, a second-stage optical splitter, a fiber Bragg grating sensor, a photodiode, an amplifier circuit, an ADC chip, and a main control FPGA / microcontroller;

[0038] The MG-Y tunable semiconductor laser is connected to the first-stage optical splitter, and the first-stage optical splitter is connected to a plurality of second-stage optical splitters ( Figure 1 a total of three second-stage optical splitters are connected in the figure), and each second-stage optical splitter is sequentially connected to a plurality of fiber Bragg grating sensors ( Figure 1 in the figure, one second-stage optical splitter is sequentially connected to two fiber Bragg grating sensors), each second-stage optical splitter is connected to a photodiode, each photodiode is connected to an amplifier circuit, each amplifier circuit is connected to an ADC chip, and a plurality of ADC chips are all connected to the main control FPGA / microcontroller; As Figure 1 shown in the figure, three photodiodes, three amplifier circuits, three ADC chips and the main control FPGA / microcontroller together form a data acquisition card.

[0039] The working process of the demodulation system of the present invention is as follows:

[0040] After the demodulation system receives the start demodulation instruction, the wavelength of the MG-Y tunable semiconductor laser needs to be controlled and tuned by 5 currents for output. The 5 currents include the right current, the left current, the phase current, the SOA current, and the Gain current. During the grating demodulation process, 5 currents are required to control the MG-Y tunable semiconductor laser to output a wavelength, so that the output wavelength steps from the starting wavelength to the ending wavelength stably at a fixed step size and fixed power.

[0041] The output wavelength passes through the first-stage optical splitter and the second-stage optical splitter and enters the external fiber Bragg grating sensor. When the output wavelength matches the fiber Bragg grating central wavelength of the fiber Bragg grating sensor, the output wavelength light is reflected back to the second-stage optical splitter by the Bragg grating of the fiber Bragg grating sensor, and is output to the photodiode through another port of the second-stage optical splitter. The photodiode converts the reflected optical signal into a current signal, which enters the ADC chip through the amplifier circuit, and then the main control FPGA / microcontroller performs data acquisition and demodulates the fiber Bragg grating central wavelength. During the wavelength demodulation process, optical wavelength scanning is performed, including the initial scan, the second and subsequent scans.

[0042] (1) Initial scan

[0043] During the initial scan, the master FPGA / microcontroller controls the right current, left current, phase current, SOA current, and Gain current, so that the right current, left current, phase current, SOA current, and Gain current control the MG-Y tunable semiconductor laser to perform optical wavelength scanning. The wavelength scanning is carried out from the starting wavelength in fixed steps until the termination wavelength is scanned. When the difference between the wavelength output by the MG-Y tunable semiconductor laser and the central wavelength of the fiber grating is less than the threshold (i.e., the output wavelength is close to the central wavelength of the fiber grating), the reflected light of the fiber grating sensor begins to increase. When the output wavelength of the MG-Y tunable semiconductor laser is exactly equal to the central wavelength of the fiber grating, the reflected light of the fiber grating sensor reaches the strongest. When the output wavelength of the MG-Y tunable semiconductor laser continues to increase, the reflected wavelength of the fiber grating sensor begins to decrease. At this time, the corresponding photodiode current and ADC chip voltage achieve a process of increasing first and then decreasing, so as to collect the fiber grating reflection spectrum and complete the initial scan.

[0044] After obtaining the fiber grating reflection spectrum in the initial scan, the central wavelength of the fiber grating is calculated according to the centroid method, and the formula is as follows:

[0045]

[0046] Among them, WAVE 中心波长 represents the central wavelength of the fiber grating to be calculated; a to b represent the sequence numbers from the start of scanning corresponding to the wavelength points recorded when the spectral amplitude exceeds the demodulation threshold; RANGE n represents the spectral peak value of the reflected wavelength corresponding to each point when scanning from point a to b; WAVE step represents the scanning step of the MG-Y tunable semiconductor laser; WAVE start represents the actual wavelength value when the MG-Y tunable semiconductor laser starts scanning.

[0047] (2) Scans for the second time and subsequent scans

[0048] After the initial scan is completed, the demodulation system has calculated the current central wavelength of the fiber grating. Since a continuous scan from the initial wavelength to the termination wavelength in fixed steps is required during the initial scan, the scanning and calculation processes are particularly numerous. In the scans for the second time and subsequent scans of the present invention, it is not necessary to perform a full scan, and only the wavelength range of ±200 pm (a set wavelength, which can be adjusted as needed and is not limited here) around the central wavelength of the fiber grating calculated in the initial scan (this range is determined by the bandwidth of the fiber grating. Generally, the scanning range is taken as the -3 dB bandwidth of the fiber grating) needs to be scanned, and the scanning is carried out at the set step size, and the central wavelength of the fiber grating can be calculated according to the centroid method of the initial scan based on the wavelength sequence number and the reflected spectrum value during the scan.

[0049] Such asFigure 2 As shown in the figure, the present invention also provides a method for demodulating the central wavelength of a fiber grating based on edge filtering. Based on the above-mentioned system for demodulating the central wavelength of a fiber grating based on edge filtering, the method specifically includes the following steps:

[0050] S1. The right current, left current, phase current, SOA current, and Gain current control the MG-Y tunable semiconductor laser to output a WAVE start wavelength;

[0051] S2. The right current, left current, phase current, SOA current, and Gain current control the output of the current wavelength + WAVE step wavelength;

[0052] S3. Determine whether the output wavelength is less than the termination wavelength;

[0053] S4. If the output wavelength is less than the termination wavelength, return to step S2;

[0054] S5. If the output wavelength is greater than or equal to the termination wavelength, calculate the central wavelength of the fiber grating using the centroid method;

[0055] S6. The right current, left current, phase current, SOA current, and Gain current control the output of the central wavelength of the nth grating - the set wavelength;

[0056] S7. The right current, left current, phase current, SOA current, and Gain current control the output of the current wavelength + WAVE step wavelength;

[0057] S8. Determine whether the output wavelength is less than the central wavelength of the nth grating + the set wavelength;

[0058] S9. If the output wavelength is less than the central wavelength of the nth grating + the set wavelength, return to step S7;

[0059] S10. If the output wavelength is greater than or equal to the central wavelength of the nth grating + the set wavelength, calculate the central wavelength of the nth grating using the centroid method;

[0060] S11. Return to step S6 until all wavelengths are calculated.

[0061] The present invention abandons the full-band scanning for each time of the MG-Y tunable semiconductor laser. Only during the first scan after the demodulation system receives an instruction, a large-step and full-band scan is performed to roughly determine the central wavelength of each fiber grating. During subsequent scans, a small-step precise scan near the central wavelength is performed. In summary, in the system for grating demodulation based on the MG-Y tunable semiconductor laser, the present invention combines full-band scanning and local scanning, and combines large-step coarse scanning and small-step precise scanning, so as to improve the demodulation speed and demodulation accuracy, reduce the speed requirement of the analog chip, and thus reduce the cost.

[0062] The combination of full-band scanning and local scanning makes the scanning speed of the system faster. Without upgrading the hardware, the demodulation speed can be increased a lot; the combination of large-step coarse scanning and small-step precise scanning can greatly improve the demodulation accuracy of the demodulation system. For example, the demodulation range of the demodulator is from 1527 nm to 1568 nm, the scanning step of the demodulator is 10 pm. In engineering applications, generally, a demodulator with a 41-nm band is connected to 4 sensors, and the sensor bandwidth is 400 nm. The traditional scanning method is to perform a step-by-step scan with a step of 10 pm from 1527 nm to 1568 nm. The time for threshold judgment once during scanning is 10 ns, and the central wavelength calculation is performed after scanning. The time for the central wavelength centroid method calculation is 500 ns. It is assumed that the hardware requires 100 ns for one step change.

[0063] Then the time required for one full-band demodulation by the traditional demodulation method is:

[0064]

[0065] The frequency is about 2207 Hz.

[0066] Based on the above hardware architecture, adopting the demodulation method of the present invention, the step of the small-step precise scan is set to 1 pm. Then the time required for one full-band demodulation is:

[0067]

[0068] The frequency is about 5618 Hz.

[0069] Therefore, the demodulation speed of the demodulation method proposed by the present invention is about 2.5 times higher than that of the traditional method. The demodulation accuracy of the centroid method is linearly related to the scanning step during demodulation. The demodulation accuracy of the demodulation method proposed by the present invention is 10 times higher than that of the traditional demodulation method.

[0070] It should be noted that in this text, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, apparatus, article or method including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent in such a process, apparatus, article or method. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of additional identical elements in the process, apparatus, article or method including such an element.

[0071] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be equally included in the patent protection scope of the present invention.

Claims

1. An optical fiber grating central wavelength demodulation system based on edge filtering, characterized in that It includes an MG-Y tunable semiconductor laser, a first-stage optical splitter, a second-stage optical splitter, a fiber Bragg grating sensor, a photodiode, an amplifier circuit, an ADC chip, and a main control FPGA / microcontroller; The MG-Y tunable semiconductor laser is connected to the first-stage optical splitter. The first-stage optical splitter is connected to multiple second-stage optical splitters. Each second-stage optical splitter is sequentially connected to multiple fiber Bragg grating sensors. Each second-stage optical splitter is connected to a photodiode. Each photodiode is connected to an amplifier circuit. Each amplifier circuit is connected to an ADC chip. Multiple ADC chips are connected to the main control FPGA / microcontroller; The MG-Y tunable semiconductor laser is controlled by right current, left current, phase current, SOA current, and Gain current for wavelength output, so that the output wavelength steps from the starting wavelength to the ending wavelength stably at a fixed step size and fixed power. The output wavelength passes through the first-stage optical splitter and the second-stage optical splitter and enters the external fiber Bragg grating sensor. When the output wavelength matches the center wavelength of the fiber Bragg grating of the fiber Bragg grating sensor, the output wavelength light is reflected back to the second-stage optical splitter by the Bragg grating of the fiber Bragg grating sensor and is output to the photodiode through another port of the second-stage optical splitter. The photodiode converts the reflected light signal into a current signal, which enters the ADC chip through the amplifier circuit. Then, the main control FPGA / microcontroller performs data acquisition and demodulates the center wavelength of the fiber Bragg grating. During the wavelength demodulation process, optical wavelength scanning is performed, including initial scanning, secondary and subsequent scans.

2. The fiber grating central wavelength demodulation system based on edge filtering according to claim 1, characterized in that During the initial scan, the main control FPGA / microcontroller controls the right current, left current, phase current, SOA current, and Gain current, so that the right current, left current, phase current, SOA current, and Gain current control the MG-Y tunable semiconductor laser to perform optical wavelength scanning, and the wavelength is scanned from the starting wavelength at a fixed step until the ending wavelength is scanned.

3. The fiber grating central wavelength demodulation system based on edge filtering according to claim 2, wherein When the difference between the wavelength output by the MG-Y tunable semiconductor laser and the center wavelength of the fiber Bragg grating is less than the threshold, the reflected light of the fiber Bragg grating sensor begins to increase. When the output wavelength of the MG-Y tunable semiconductor laser is exactly equal to the center wavelength of the fiber Bragg grating, the reflected light of the fiber Bragg grating sensor reaches the strongest. When the output wavelength of the MG-Y tunable semiconductor laser continues to increase, the reflected wavelength of the fiber Bragg grating sensor begins to decrease. At this time, the corresponding photodiode current and ADC chip voltage realize a process of first increasing and then decreasing, so that the fiber Bragg grating reflection spectrum is collected and the initial scan is completed.

4. The fiber grating central wavelength demodulation system based on edge filtering according to claim 3, wherein After obtaining the fiber Bragg grating reflection spectrum, the center wavelength of the fiber Bragg grating is calculated according to the centroid method, and the formula is as follows: Among them, WAVE 中心波长 represents the central wavelength of the fiber grating to be calculated; a to b represent the sequence numbers from the start of scanning corresponding to the wavelength points recorded when the spectral amplitude exceeds the demodulation threshold; RANGE n represents the spectral peak value of the reflected wavelength corresponding to each point when scanning from point a to point b; WAVE step represents the scanning step of the MG-Y tunable semiconductor laser; WAVE start represents the actual wavelength value when the MG-Y tunable semiconductor laser starts scanning.

5. The fiber grating central wavelength demodulation system based on edge filtering according to claim 4, wherein After the initial scan is completed, secondary and subsequent scans begin. The range of the calculated center wavelength of the fiber Bragg grating ± the set wavelength is used, and the scan is performed at the set step size. According to the wavelength serial number and the reflection spectrum value during the scan, the center wavelength of the fiber Bragg grating is calculated according to the centroid method of the initial scan.

6. The fiber grating central wavelength demodulation system based on edge filtering according to claim 4, characterized in that The range of the center wavelength of the fiber Bragg grating ± the set wavelength is determined according to the bandwidth of the fiber Bragg grating sensor.

7. A method for demodulating the central wavelength of a fiber Bragg grating based on edge filtering, characterized in that Based on the edge-filtering-based fiber grating central wavelength demodulation system according to any one of claims 4 to 6, the method includes: S1, right current, left current, phase current, SOA current, and Gain current control the MG-Y tunable semiconductor laser to output WAVE start wavelength; S2, Right Current, Left Current, Phase Current, SOA Current, Gain Current control the current wavelength + WAVE step Wavelength Output; S3. Determine whether the output wavelength is less than the termination wavelength; S4. If the output wavelength is less than the termination wavelength, return to step S2; S5. If the output wavelength is greater than or equal to the termination wavelength, calculate the central wavelength of the fiber grating using the centroid method; S6. Control the output of the central wavelength - set wavelength of the nth grating with right current, left current, phase current, SOA current, and Gain current; S7, Right Current, Left Current, Phase Current, SOA Current, Gain Current control current wavelength + WAVE step Wavelength output; S8. Determine whether the output wavelength is less than the central wavelength of the nth grating + set wavelength; S9. If the output wavelength is less than the central wavelength of the nth grating + set wavelength, return to step S7; S10. If the output wavelength is greater than or equal to the central wavelength of the nth grating + set wavelength, calculate the central wavelength of the nth grating using the centroid method; S11. Return to step S6 until all wavelengths are calculated.

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